1. Field of the Invention
The present invention relates generally to refrigeration cycles, and particularly to a refrigeration cycle for a vehicle air conditioner in which high-temperature gas refrigerant discharged from a compressor is directly introduced into an evaporator. The evaporator functions as a radiator for radiating heat of the gas refrigerant to improve heating performance of the air conditioner.
2. Related Art
Conventionally, when a vehicle air conditioner is operated in a heating mode in winter, high-temperature engine coolant for cooling an engine of a vehicle flows through a heating heat exchanger. The heat exchanger performs heat exchange between air and engine coolant so that air is heated. However, when a temperature of engine coolant is relatively low, air passing through the heat exchanger may insufficiently heated and a temperature of air blown into a passenger compartment of the vehicle may be lowered. Thus, heating performance of the air conditioner may be declined.
JP-A-5-272817 discloses a refrigeration cycle for a vehicle air conditioner. In the refrigeration cycle, when a temperature of engine coolant is relatively low at a time of starting the engine, high-temperature gas refrigerant discharged from a compressor directly introduced into an evaporator through a bypass passage while bypassing a condenser. The evaporator radiates heat of gas refrigerant toward air to heat air.
Further, excess or shortage of an amount of refrigerant circulating through the cycle is detected according to a cycle high pressure of refrigerant detected in the vicinity of an outlet of the compressor. When the amount of refrigerant circulating through the cycle is judged excessive, refrigerant is supplied to the condenser. When the amount of refrigerant circulating through the cycle is judged insufficient, refrigerant staying in the condenser is forced to be discharged and introduced to the evaporator. Refrigerant staying in the condenser is returned to the evaporator by operating the compressor while an inlet of the condenser is opened and an inlet of the bypass passage is closed, that is, in a normal cooling mode.
Generally, a separator which separates gas-liquid two-phase refrigerant into gas refrigerant and liquid refrigerant and stores liquid refrigerant therein is connected to an outlet of the condenser. In the cooling mode, high-pressure liquid refrigerant discharged from the separator is decompressed by a thermostatic expansion valve, and low-pressure refrigerant decompressed by the expansion valve is evaporated by the evaporator. An opening degree of the expansion valve is adjusted so that refrigerant discharged from the evaporator has a predetermined degree of superheat such as 10.degree. C., under normal operation conditions in the cooling mode such as a cycle low pressure is set to 2-5 kg/cm.sup.2 G.
However, when outside air has an extremely low temperature such as -10.degree. C. or lower, since a temperature sensor of the expansion valve also has a temperature substantially the same as a temperature of outside air at a time of starting a heating mode, a pressure of refrigerant in the temperature sensor is decreased to an extremely low value in comparison with that in the cooling mode. As a result, an opening degree of the expansion valve becomes small, and the expansion valve may be eventually closed when the temperature of outside air is further decreased. Therefore, even when refrigerant staying in the condenser is forced to be returned to the evaporator, refrigerant may not be sufficiently returned to the evaporator.